1. Technical Field
The present invention relates to a magnetoresistive-effect thin film using the giant magneto-resistivity. The present invention also relates to a magnetoresistive-effect element and a magnetoresistive-effect magnetic head fabricated with the magnetoresistive-effect thin film using the giant magneto-resistivity.
2. Prior Art
Conventionally, there is a widely used magnetoresistive-effect magnetic head (hereafter referred to as the MR head) which uses magnetoresistance of a magnetoresistive-effect element (hereafter referred to as the MR element) to read signals recorded on a magnetic storage medium.
The MR element is a type of resistance element and varies electrical resistance according to an external magnetic field. The MR head reads magnetic signals recorded on a magnetic storage medium by using the fact that the MR element's electrical resistance varies with a signal magnetic field from the magnetic storage medium.
In recent years, there is an increasing need for a small, large-capacity magnetic storage medium. For example, a technique such as narrowing a recording track width accelerates the trend towards a higher recording density of magnetic storage media.
With respect to the MR head, a particular attention is paid to utilize an MR thin film based on the giant magneto-resistivity (GMR) instead of a conventional MR thin film using the anisotropic magneto-resistivity (AMR). The purpose is to prevent a signal output from decreasing due to narrowed tracks on the magnetic storage medium.
Of MR thin films using the giant magneto-resistivity, a spin-valve film comprises an antiferromagnetic layer, two ferromagnetic layers, and a non-magnetic layer. The non-magnetic layer is formed between the two ferromagnetic layers. The antiferromagnetic layer is formed adjacent to one of the two ferromagnetic layers. The ferromagnetic layer in contact with the antiferromagnetic layer is used as a pinned layer. The other ferromagnetic layer is used as a free layer. The free layer magnetization varies with an external magnetic field. The external magnetic field is detected according to a difference in magnetization directions for the pinned layer and the free layer.
The MR head tends to lower an MR element for improving outputs. Consequently, this thins the free layer in the spin-valve film. However, thinning the free layer makes the MR element asymmetric. For decreasing this asymmetry, it is only necessary to decrease demagnetization applied to the layer from the pinned layer. For this purpose, it is only necessary to decrease a magnetic moment for the pinned layer. For decreasing the pinned layer's magnetic moment, it is only necessary to thin the pinned layer. However, thinning the pinned layer decreases an MR head output.
From the viewpoint of increasing recording densities, the spin-valve film tends to thin the free layer for reducing its magnetic moment. However, thinning the free layer may degrade magnetoresistance.
To solve this, it is proposed to use a structure comprising a pair of antiferromagnetically combined ferromagnetic layers for the pinned layer or the free layer by forming a non-magnetic layer between a pair of ferromagnetic layers for the spin-valve film. This structure is called the layered ferrimagnetic structure (VS. Speriousu et.al; The 1996 IEEE INTERMAG,AA-04). The layered ferrimagnetic structure provides a 3-layer structure by forming a non-magnetic layer between a pair of ferromagnetic layers. Adjusting the non-magnetic layer thickness antiferromagnetically binds a pair of ferromagnetic layers. The above-mentioned non-magnetic layer is generally formed of Ru, Rh, Ir, Re, and the like.
When the pinned layer uses this layered ferrimagnetic structure, a pair of antiferromagnetically coupled ferromagnetic layers decreases an apparent magnetic moment for the pinned layer. Even if the free layer is thin, demagnetization applied to the free layer decreases. This makes it possible to improve the free layer's sensitivity against an external magnetic field without excessively thinning the pinned layer.
Examples of the spin-valve film which uses the layered ferrimagnetic structure for the free layer are reported in studies by A. Veloso et al. (IEEE Trans. Magn. Vol. 35, No. 5, P2568-2570, September 1999) and the like. Since a pair of ferromagnetic layers is antiferromagnetically coupled in the free layer, an apparent magnetic moment for the free layer can be decreased by maintaining the thickness of one magnetic layer associated with magnetoresistive-effect and adjusting the thickness of the other magnetic layer not associated therewith. Accordingly, it is possible to improve the free layer's sensitivity against an external magnetic field without excessively thinning the free layer.
On the other hand, improvement of the magnetoresistive-effect change necessitates consideration of increasing a probability that electrons scatter depending on spins in the spin-valve film and improving the magnetoresistive-effect change in the spin-valve film. Incidentally, this scattering of electrons is hereafter referred to as spin-dependent scattering.
When the layered ferrimagnetic structure is applied to the free layer, however, there is provided a new magnetic layer not associated with magnetoresistive-effect. A shunt loss may occur in this magnetic layer. There is the problem that a resulting magnetoresistive-effect change is not as high as expected despite the use of the layered ferrimagnetic structure for the free layer.
Japanese Patent Application Laid-Open Publication No. 11-8424 discloses an example of the spin-valve film which increases the probability of spin-dependent scattering as mentioned above. According to this example, a metal layer which easily causes mirror reflection is formed adjacent to the pinned layer and the free layer. However, many such metal materials provide low resistivity. Consequently, a current shunts to the layer of such metal easily causing mirror reflection, raising the possibility of decreasing MR head output.
Japanese Patent Application Laid-Open Publication No. 11-168250 and the study by W. F. Egelhoff et. al. (J. Appl. Phys. 82(12), Dec. 15, 1997) provide an example of the spin-valve film which increases the probability of spin-dependent scattering as mentioned above. According to this example, an antiferromagnetic film of oxide is formed in the spin-valve film. However, the antiferromagnetic film formed of oxide does not provide a sufficient exchange coupling force with a ferromagnetic film used as the pinned layer. The antiferromagnetic film formed of oxide lacks thermal stability and does not ensure the reliability as an antiferromagnetic film compared to an antiferromagnetic film formed of presently used ordered metal.
Further, the study by Y. Kamiguchi et. al. (The 1999 IEEE INTERMAG, DB-1) provides an example of the spin-valve film which increases the probability of spin-dependent scattering as mentioned above. According to the example, the spin-valve film contains a metal oxide layer formed in the middle of a pinned layer. However, forming a metal oxide layer in the middle of the pinned layer thickens it. Thickening the pinned layer increases demagnetization applied to the free layer.
Conventionally, the spin-valve film uses a non-magnetic layer with a thickness of 2.4 to 3.2 nm formed between the pinned layer and the free layer. From the viewpoint of improving the magnetoresistive-effect change, it is desirable to make the non-magnetic layer as thin as possible. However, thickening the non-magnetic layer excessively increases an inter-layer coupling field between the pinned layer and the free layer. A change in the free layer's magnetization direction may also change the pinned layer's magnetization direction.